"Of the three muddies, one had horrible looking ulcers on the top of its shell and when I cut it open its gills appeared to be diseased.

"I showed my mum, who has been fishing for mud crabs all of her life and she said she had never seen anything like it."

Mr Edwards, who has lived at Koongal for the past seven years, said he goes crabbing every couple of months.

"They were caught at the back of the mangroves at Port Alma but we have no idea what has caused this," Mr Edwards said.

"I am no expert but we are at a loss as to what happened because the water is clean and there is no heavy industry with discharge going into the water - not that we know of."

Pictures of the diseased crab, which Mr Edwards emailed to The Morning Bulletin, were forwarded to Queensland Fisheries for their pathologist to view and to see if she could determine what had caused the disfiguration.

After examining the photos the pathologist said: "The carapace of the crab shows perforated and non-perforated rust spots adjacent to the left-hand eye and what may be other shell damage.

"Without a sample of the crab to examine, it is difficult to know what caused the shell erosion.

"However, it appears consistent with shell abnormalities that have been seen before on mud crabs in various locations in Queensland.

"Studies since the 1990s have identified rust spot and shell erosion on crabs in Queensland, including the Fitzroy River, and so it is known to occur.

"The crab also appears to have small commensal barnacles on its gills, which is not uncommon in crabs,'' she said.

"It is difficult to identify if there are other issues in the gills from the photo.

"Any seafood, regardless of where it was caught or purchased, that shows signs of damage, deterioration or ill health, should not be eaten.''

Meanwhile, the Newman Government has released a report on the water quality, fish and human health in and around Gladstone Harbour.